Constraints on the quantitative assessment of in-reservoir biodegradation using compound-specific stable carbon isotopes

With the increasing interest and technical availability of compound-specific isotope analysis, this analytical method has successfully been applied to characterise microbial degradation processes of hydrocarbons, and in petroleum geochemistry. For quantification of the biodegradation processes, isotopic fractionation is evaluated using the Rayleigh equation. The Rayleigh model can also be successfully used to quantify biodegradation of light hydrocarbons in petroleum reservoirs. We demonstrate here the reliability of the Rayleigh approach for evaluation of crude oil biodegradation in two different petroleum systems and discuss the geochemical implications for processes such as charging and mixing and the limitations of the method.